Infrared spectra of mass-selected Mg+-H2 and Mg +-D2 complexes

Rotationally resolved infrared spectra of Mg+-H2 and Mg+-D2 are recorded in the H-H (4025-4080 cm-1) and D-D (2895-2945 cm-1) stretch regions by monitoring Mg + photofragments. The Vhh and Vdd transitions of Mg+-H2 and Mg+-D2 are red-shifted by 106.2 ± 1.5 and 76.0± 0.1 cm-1 respectively from the fundamental vibrational transitions of the free H2 and D 2 molecules. The spectra are consistent with a T-shaped equilibrium structure in which the Mg+ ion interacts with a slightly perturbed H2 or D2 molecule. From the spectroscopic constants, a vibrationally averaged intermolecular separation of 2.716 A° (2.687 A°) is deduced for the ground state of Mg+-H2 (Mg +-D2), decreasing by 0.037 A° (0.026 A°) when the H2 (D2) subunit is vibrationally excited.